CN113167682A

CN113167682A - Method, computer program and electronic storage medium for detecting particles or aerosols in a flowing fluid

Info

Publication number: CN113167682A
Application number: CN201980082431.8A
Authority: CN
Inventors: R·鲁萨诺夫
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2018-12-13
Filing date: 2019-10-23
Publication date: 2021-07-23
Also published as: EP3894824A1; US20220026338A1; KR20210098471A; WO2020119990A1; DE102018221700A1

Abstract

A method for detecting particles or aerosols in a flowing fluid by using the principle of laser induced incandescent light, the method comprising the steps of: a. focusing laser light emitted from a laser into a light spot, b. guiding a fluid containing particles or aerosol through the light spot, c. detecting thermal radiation emitted from the light spot by means of a detector, d. analyzing a parameter provided by the detector and characterizing the detected thermal radiation over a plurality of time intervals, wherein the duration of the time intervals depends on the velocity of the fluid.

Description

Method, computer program and electronic storage medium for detecting particles or aerosols in a flowing fluid

Technical Field

The invention relates to a method for detecting particles or aerosols in a flowing fluid by using the principle of laser induced incandescent light (laserinduzierten inkendenzez), a computer program and an electrical storage medium according to the preambles of the accompanying claims.

Background

The principle of laser-induced incandescent Light (LII) has long been known for detecting nanoparticles in gases, for example in air, and is also used intensively, for example, for characterizing the combustion process of "glass-made" engines in laboratories, or for exhaust gas characterization in laboratory environments. In this case, the particles, soot particles, are heated to several thousand degrees celsius by nanosecond pulses of a high-power laser, so that the soot particles emit a large amount (significant) of thermal radiation or heat radiation. This thermally induced optical radiation of the particles is measured by means of a photodetector. The differentiation of the signal of small particles from the so-called "background signal" caused by thermal effects and/or signal noise is a challenge here.

Disclosure of Invention

The problem on which the invention is based is solved by a method having the features of claim 1 and by a computer program and an electronic storage medium having the features of the parallel claims. Advantageous embodiments are specified in the dependent claims.

The method according to the invention is used for detecting particles or aerosols in a fluid, for example in an exhaust gas. It works by using the principle of laser induced incandescent Light (LII). Here, the particles are first heated to several thousand degrees by partially absorbing the laser light emitted from the laser and focused with sufficiently high intensity in a Spot (Spot), i.e. a volume range with a minimum dimension in the μm range. The hot particles emit characteristic thermal radiation (incandescent or thermionic radiation) according to the planck's law of radiation, which is used as a measurement signal and is received by a detector. The spectrum of this thermally emitted light (thermal radiation) is typically relatively broad with a maximum in the red range (about 750 nm).

For this purposeThe use of an optical element arranged in the beam path of the laser, which optical element is constructed and arranged for focusing the laser light emitted from the laser in a very small spot. When based on 10¹³/m³With a particle concentration of, for example, 10 μm, it can be assumed that only one particle always flies over (durchfligen) the light spot (intrinsic single-particle detectability) at a given time. The detector is arranged and disposed such that it detects thermal radiation emitted from the light spot. A cost-effective semiconductor laser diode can be used as the laser. The detection of thermal radiation can be realized, for example, by means of sensitive photodiodes or multi-pixel photon counters (MPPCs).

In particular, the method according to the invention comprises at least the following steps:

a. the laser light emitted from the laser is focused into a spot,

b. directing a fluid containing particles or aerosol through the light spot,

c. the thermal radiation emitted from the light spot is detected by means of a detector,

d. the parameter provided by the detector and characterizing the detected thermal radiation is evaluated in a plurality of time intervals, wherein the duration of the time intervals depends on the speed of the fluid.

Here, the present invention utilizes the following facts: the particles or aerosols have a typical time-of-flight through the laser spot, which depends on the known and constant spot size and in particular on the variable speed of the fluid in which they are located. This makes it possible to predict the possible time duration during which the signal provided by the detector changes on the basis of the detection of thermal radiation. Therefore, the signal analysis processing can be limited to this duration, so that the "background signal noise" present before and after can be masked and thus have a small influence.

The object of the invention is therefore a method for extending the signal evaluation process, in which information about the fluid speed (for example from an engine controller of an internal combustion engine) is used to: a time interval (particle detection interval) during which a parameter characterizing the detected thermal radiation (e.g. the variation of the intensity over time) is evaluated is controlled as a function of the velocity of the fluid and thus the signal-to-noise ratio is optimized. In this case, the time interval is shorter if the fluid speed is high, compared to if the fluid speed is low.

The method according to the invention allows the quantitative and mass concentration of particles or aerosols in a flowing fluid to be measured, in particular soot particles in the exhaust gases of diesel and gasoline vehicles. The ability to perform single particle detection in the test volume is explicitly included here, so that the particle size can be determined from the measurement data. The method according to the invention can be used for OBD monitoring (On Board diagnostics) of the state of the particle filter. The particle sensor operated by means of the method according to the invention has a short response time and is ready almost immediately after activation.

Just in gasoline vehicles, the scalability of the particle number and the immediate readiness immediately after the vehicle is started are very important, since during a cold start very fine particles (of low quality, high number) are produced which are mostly typically emitted in motor vehicles with gasoline internal combustion engines.

The invention allows an improvement or optimization of the ratio between the actual signal and the signal noise, so that even very small soot particles can be reliably detected. The reduction of the validation limit (Nachweisgrenze), for example to particle sizes below 23nm, is possible in particular by the process according to the invention. Finally, the computational overhead is reduced since the method according to the invention enables the use of simplified analysis processing algorithms.

In one embodiment of the invention, it is provided that at least some of the time intervals overlap. This allows for a seamless evaluation of the parameters characterizing the detected thermal radiation. The time interval can thus be a type of "sliding window", that is to say a time interval during which the variable supplied by the detector is evaluated and compared with the expected background noise, wherein the time interval is "shifted" forward with a defined time raster, for example every 1 μ s, so that the temporally last part of the processing variable is always evaluated during the time interval.

In one embodiment of the invention, it is provided that the duration of the time interval is greater than the desired FWHM of the parameter characterizing the heat emission, in particular approximately 1 to 2 times the desired FWHM, more preferably approximately 1.5 times the desired FWHM. FWHM is to be understood as meaning "Full Width at Half Maximum (Full Width at Half Maximum)" or "Half-value Width (Halbwertsbreite)" in which the difference between two parameter values (argmentwerten) is referred to for which the value of the function drops to Half the Maximum value. This possibility is achieved in this way: in the event of a particle detection, the entire relevant region of the variation process of the parameter characterizing the detected thermal radiation is evaluated.

The duration of the time interval during which the variable supplied by the detector is compared with the expected background and a determination is made as to whether a particle is detected or not is therefore adapted to the expected FWHM of the variable supplied by the detector, which is determined on the basis of the velocity of the fluid. The duration of the time interval can be, for example, one or two times the desired FWHM. The matching of the time intervals or the duration of the "evaluation Windows" serves to not unnecessarily collect background noise around the expected signal in the case of a detected particle, which would deteriorate the signal-to-noise ratio.

In one embodiment of the invention, it is provided that the overlapping time periods of two adjacent or successive time intervals correspond to at least half of the duration of the time intervals. This allows a reliable evaluation of the entire course of the change of the variable supplied by the detector.

In one embodiment of the invention, it is provided that a particle is detected if the parameter characterizing the thermal radiation or the parameter determined from the thermal radiation reaches at least a limit value within a time interval. This can be implemented simply in terms of programming.

The limit value can depend on the desired background noise. In this way the "sensitivity" can be matched to the desired background noise.

In one embodiment of the invention, it is provided that at least some of the successive time intervals do not overlap, but preferably directly adjoin one another. This can also be implemented very simply in terms of programming. The parameter characterizing the thermal radiation is "collected" at temporally fixed intervals, which may have, for example, a duration of 0.5 times the FWHM.

In one embodiment of the invention, it is provided that a particle is detected when the quantity characterizing the thermal radiation or the quantity determined from the thermal radiation reaches at least one limit value or a plurality of different limit values in at least two time intervals directly following one another. In this way, the detection of particles can be displayed particularly simply. In this case, the limit value or the limit values can again depend on the desired background noise.

In one embodiment of the invention, it is provided that the variable characterizing the thermal radiation is a continuous variable and is preferably integrated in the context of the evaluation process from the continuous variable over the time interval. This is suitable, for example, where the detector is a photodiode.

In one embodiment of the invention, the parameter characterizing the thermal radiation comprises a discontinuous parameter, in particular a plurality of pulse-like signals. This is suitable where the detector is an MPPC. In the context of the evaluation process, a plurality of pulse-like signals can then be extracted in a time interval.

It will be appreciated that the types of time intervals described above (overlapping/non-overlapping) can also be combined with each other, i.e. can be implemented in a mixed form.

In one embodiment of the invention, the speed of the fluid is determined from the FWHM of the preferably large particle, and this determined speed is then used to determine the length of the time interval for the detection of small particles. In the case of large particles, the SNR (signal-to-noise-ratio) is particularly advantageous.

The invention also comprises a computer program, which is programmed for carrying out the method according to any one of the preceding claims, and an electrical storage medium for an evaluation device, in particular for use in an exhaust system of an internal combustion engine, in particular an ASIC, on which a computer program for carrying out the method is stored, which is programmed for carrying out the method.

Drawings

Embodiments of the invention are explained below with reference to the attached figures. Shown in the drawings are:

fig. 1 shows the measuring principle based on laser-induced incandescent light, which is used in the present invention by using a detector in the form of an exemplary photodiode;

fig. 2 shows a theoretical structure of a particle sensor using the measurement principle shown theoretically in fig. 1;

FIG. 3 shows a block diagram illustrating the structure of the particle sensor of FIG. 2;

FIG. 4 shows a more detailed illustration of the structure of the particle sensor of FIG. 3, including a representation of a flowing fluid in which particles are present;

figure 5 shows a graph of the course over time of a quantity characteristic of the detected thermal radiation provided by the detector of the particle sensor of figure 4 at a first velocity of the flowing fluid together with a first type of analytical treatment-time interval,

FIG. 6 shows a graph similar to FIG. 5 with a second velocity of the flowing fluid, the second velocity being higher than the first velocity;

FIG. 7 shows a graph similar to FIG. 5 with a second type of analytical process-time interval at a first velocity of the flowing fluid;

FIG. 8 shows a graph similar to FIG. 7 with a second velocity of the flowing fluid, the second velocity being higher than the first velocity;

FIG. 9 shows a graph similar to FIG. 5, but of a different type than the parameters provided by the detector; and

fig. 10 shows a flow chart of a method for detecting particles.

Functionally equivalent elements and regions have the same reference numerals in the following description.

Detailed Description

Fig. 1 shows the measurement principle based on laser induced incandescent Light (LII). A high intensity laser 10 is directed at particles 12, such as soot particles in the exhaust stream of an internal combustion engine (not shown). The intensity of the laser 10 is so high that the energy of the laser 10 absorbed by the particles 12 heats the particles 12 to several thousand degrees celsius. As a result of the heating, the particles 12 emit radiation 14, also referred to as LII light, spontaneously and substantially without preferential direction, largely in the form of thermal radiation. Thus, a portion of the radiation 14 emitted in the form of thermal radiation is also emitted in a direction opposite to the direction of the incident laser light 10.

Fig. 2 schematically shows the principle structure of one embodiment of the particle sensor 16. The particle sensor 16 has a CW laser module 18 (CW: continuous wave), the preferably parallel laser light 10 of which is focused onto a very small spot 22 by means of at least one optical element 20 arranged in the beam path of the CW laser module 18. A light spot is understood here as a volume element having a very small size in the μm range. The optical element 20 preferably includes a lens 24. Only in the volume of the spot 22 does the intensity of the laser 10 reach the high values required for laser induced incandescence.

The size of the spot 22 lies in the range of a few micrometers, in particular up to 200 μm, so that the particles 12 excited to traverse the spot 22 radiate the radiation power which can be analytically processed, whether by laser-induced incandescent light or by chemical reactions (in particular oxidation). As a result, it can be considered that: always at most one particle 12 is in the light spot 22 and the instantaneous measurement signal of the particle sensor 16 comes from only this at most one particle 12.

The measurement signal is generated by a detector 26, which is arranged in the particle sensor 16 in such a way that it detects the radiation 14, in particular the thermal radiation, emitted from the particles 12 flying over the light spot 22. In this regard, the measurement signal provided by the detector 26 is a parameter that characterizes the detected thermal radiation. For this purpose, the detector 26 preferably has at least one photodiode 26.1 which detects the thermal radiation and enables quantification (intensity as a function of time). Thereby enabling single particle measurement: the single particle measurement enables extraction of information about the particle 12, such as size and velocity. Examples of photodiodes 26.1 include silicon photomultipliers (silicon photomultipliers) or SPAD diodes (single-photon avalanche diodes), which are cost-effective. Alternatively, the detector may also include MPPC (Multi-Pixel-Photon-Counter).

As a result, it is already possible to detect light signals which are generated by particularly small particles and are therefore extremely small, for example, formed by several tens of photons. Thus, the size of the particles, which can be just still verified, is reduced to the lower limit of at most 10 nm.

It is entirely possible to modulate or switch the laser of the laser module 18 on and off (duty cycle < 100%). Preferably, however, the laser of the laser module 18 is a CW laser. This enables the use of cost-effective semiconductor laser elements (laser diodes), which makes the entire particle sensor 16 inexpensive and greatly simplifies the manipulation of the laser module 18 and the evaluation of the measurement signals. But does not preclude the use of pulsed lasers.

Fig. 3 shows a block diagram of one possible implementation of the particle sensor 16. First, a laser module 18 emitting laser light 10 is seen. The laser light 10 is first shaped (formen) into a parallel beam by a lens 29, which passes through a beam splitter, for example in the form of a beam splitter or dichroic mirror 30. From there, the parallel beam reaches the optical element 20 or the lens 24 and further reaches the spot 22 in a focused form.

The thermal radiation 14 (dashed arrow) of the particles 12 excited by the laser light 10 in the light spot 22 passes back again through the lens 24 to the dichroic mirror 30, where it is deflected here, for example, by 90 °, passes through the focusing lens 31 and passes through the filter 32 (which does not have to be present) to the photodiode 26.1 of the detector 26 (it is also conceivable in principle that the thermal radiation passes first through the filter and then through the focusing lens). Filter 32 is designed such that it filters out the wavelength of laser light 10. The interfering background is reduced by the filter 32. The embodiment with the filter 32 particularly exploits the narrow bandwidth of the laser source (e.g. laser diode) in such a way that it is filtered out just in front of the detector 26. It is also conceivable to use simple edge filters. Thereby greatly improving the signal-to-noise ratio.

Fig. 4 shows in more detail an advantageous embodiment of a particle sensor 16 which is suitable for use as a soot particle sensor in the exhaust gases of a combustion process, for example in the exhaust system of an internal combustion engine. In this connection, the exhaust gas forms an example of a particle-containing fluid flowing at a defined speed.

The particle sensor 16 has a device consisting of an outer protective tube 44 and an inner protective tube 46. Preferably, the two

protection tubes

44, 46 have a generally cylindrical (zylindorm) or prismatic shape. The bottom surface of the cylindrical shape is preferably circular, elliptical or polygonal. The cylinders are preferably arranged coaxially, with the axis of the cylinders transverse to the flow of exhaust gas 48

Is oriented. The inner protective tube 46 projects beyond the outer protective tube 44 in the direction of the axis into the flowing exhaust gas 48. At the end of the two

protective tubes

44, 46 facing away from the exhaust gas 48 flowing, the outer protective tube 44 projects beyond the inner protective tube 46. Preferably, the inner clear width (lichte Weite) of the outer protective tube 44 is much larger than the outer diameter of the inner protective tube 46, so that a first flow cross section is produced between the two

protective tubes

44, 46. The inner clear width of the inner protection tube 46 forms the second flow cross section.

This geometry results in the exhaust gas 48 entering the arrangement of the two

protective tubes

44, 46 through the first flow cross section and then changing its direction at the ends of the

protective tubes

44, 46 facing away from the exhaust gas 48, entering the inner protective tube 46 and being sucked out of the inner protective tube by the exhaust gas 48 flowing through (arrow with reference numeral 49). In this case, a laminar flow (laminar flow) is generated in the inner protective tube 46

). This arrangement of the

protective tubes

44, 46 together with the soot particle sensor 16 is fixed on or in the exhaust gas duct (not shown) transversely to the flow direction of the exhaust gas 48.

Furthermore, the soot particle sensor 16 has a laser 18 which preferably generates parallel laser light 10 as shown herein. In the beam path of the parallel laser light 10 there is a beam splitter in the form of the above-mentioned dichroic mirror 30 by way of example. The part of the laser light 10 that passes the beam splitter 30 without being diverted is focused by the optical element 20 into a very small spot 22 in the interior of the inner protective tube 46. In this spot 22, the light intensity is high enough to heat the particles 12 conveyed in the inner protective tube with the exhaust gas 48 at the velocity of the flow (arrow 49) to several thousand degrees celsius, so that the heated particles 12 emit a large amount of radiation 14 in the form of thermal radiation. The radiation 14 is, for example, in the near infrared spectral range and in the visible spectral range, without being limited to the spectral range.

A portion of the radiation 14 emitted nondirectionally in the form of thermal radiation (LII light) is detected by the optical element 20 and deflected by the beam splitter 30 and directed to the detector 26 by the lens 31 and the filter 32. This structure has particularly important advantages: only a single optical channel to the exhaust gas 48 is required, since the same optics, in particular the same optical element 20 with the lens 24, are used for generating the light spot 22 and for detecting the thermal radiation 14 emitted from the particles 12.

In the subject matter of fig. 4, the laser 18 has a laser diode 50 and a lens 52 which orients the laser light 10 emitted by the laser diode 50 in parallel. The use of a laser diode 50 represents a particularly cost-effective and simple operational possibility for generating the laser light 10. The parallel laser light 10 is focused by an optical element 20 to a spot 22.

Preferably, the soot particulate sensor 16 has a first portion 16.1 exposed to the exhaust gases and a second portion 16.2 not exposed to the exhaust gases, said second portion containing the optical components of the particulate sensor 16. The two parts are separated by a separating wall 16.3 which extends between the

protective tube

44, 46 and the optical element of the particle sensor. The wall 16.3 serves for the insulation of the sensitive optical elements from hot, chemically aggressive and "dirty" exhaust gases 32. In the separating wall 16.3, a protective window 54 is arranged in the beam path of the laser light 10, through which protective window the laser light 10 is incident on the exhaust gas 48 or the stream 49, and through which the thermal radiation 14 emitted from the spot 22 can be incident on the optical element 20 and emerging from the latter via the beam splitter 30 and the filter 32 on the detector 26. It is also conceivable that particularly sensitive components of the particle sensor, for example a laser and a detector, are arranged in a separate housing and, for the purpose of conveying laser light and/or thermal radiation to/from the optical component arranged at the exhaust gas, for example an optical waveguide in the form of one or more glass fibers is used.

The particle sensor 16 may also have an analysis processing device 56 programmed to: based on the signal of the detector 26, an evaluation of the variable provided by the detector 26 and characteristic of the detected thermal radiation is carried out. For this purpose, the evaluation unit 56 has components not shown in detail, such as a microprocessor and an electrical storage medium, on which a computer program for carrying out the method explained below is stored.

Reference is first made to fig. 5 and 6. In the figure, the variation over time t of the parameter already mentioned above and provided by the detector 26 is plotted, said parameter being characteristic of the intensity of the thermal radiation 14 detected by the detector 26. The variable supplied (hereinafter referred to as "measurement signal") has the reference number 58 in the drawing as a whole. The value of the measurement signal 58 is denoted by S. It can be seen that the measurement signal 58 is a continuous variable, which however extends in a wave shape or zigzag, which corresponds to noise.

When the particles emit thermal radiation 14, the measurement signal 58, which is otherwise kept at a constant low level, rises to an increased value (maximum Smax) and then falls again. The Full Width at Half Maximum (English: FWHM or Full Width at Half Maximum) is indicated in the figure by a double arrow with the reference numeral 60. The time intervals, which have the

reference numerals

62a, 62b and 62c, are indicated in fig. 5 and 6 by rectangular boxes. In this context, only three time intervals 62a-c are exemplarily drawn. However, there is in practice a virtually infinite sequence of time intervals. Here, the duration 64 of the time intervals 62a-c is greater than the full width at half maximum 60 herein. The duration of this time interval is here about 1.5 times the full width at half maximum 60.

It can also be seen from fig. 5 and 6 that the time intervals 62a-c overlap. The overlap period 66 between

successive time intervals

62a and 62b or 62b and 62c is constant and in this context is approximately 75% of the duration 64 of one time interval 62a-c, i.e. greater than half the duration 64 of one time interval 62 a-c.

The duration 64 of the time intervals 62a-c is variable herein. It depends on the desired full width at half maximum 60. The desired full width at half maximum 60 in turn depends on the current velocity of the flow 49 of exhaust gas 48 in the spot 22 and thus on the desired, possible residence time of the particles 12 in the spot 22. In the case of the internal combustion engine described here by way of example, the speed of the flow 49 of the exhaust gas 48 in the inner protective pipe 46 is determined or at least estimated as a function of the current operating state of the internal combustion engine, for example as a function of the current rotational speed and the current torque and as a function of the geometry of the outer protective pipe 44 and the inner protective pipe 46.

It is also conceivable to determine the desired FWHM from the signals of large, temporally adjacent particles which have a high SNR (signal-to-noise ratio) and are therefore less dependent on the method described here.

The correlation of the full width at half maximum 60 and the velocity of the flow 49 of the exhaust gas 48, and thus the correlation of the duration 64 of the time interval 62a-c and the velocity of the flow 49 of the exhaust gas 48, is such that: in the case of a relatively low velocity of the flow 49 of the exhaust gas 48, the desired full width at half maximum 60 and therefore the duration 64 is greater (fig. 5), whereas in the case of a relatively high velocity of the flow 49 of the exhaust gas 48, the desired full width at half maximum 60 and therefore the duration 64 is smaller (fig. 6).

The evaluation of the measurement signal 58 is always carried out in only one time interval 62 a-c. During the evaluation process, the measurement signal 58 is integrated, for example, in the respective time interval 62a-c, i.e. the area under the measurement signal 58 is calculated within the limits of the respective time interval 62 a-c. This integral ("integral value") is thus a quantity which is determined from the quantity characterizing the thermal radiation 14. The integrated value obtained for each time interval 62a-c is then compared with a limit value. When the integration value reaches or exceeds the limit value, the particle 12 is considered to be detected.

An alternative type of analysis process is shown in fig. 7 and 8. There, instead of overlapping time intervals, time intervals 62a-c are used which are successive to each other and directly next to each other. The measurement signal 58 is evaluated again in that the integral is determined below the measurement signal 58 in each time interval 62 a-c. A particle 12 is detected when the respective integration value in at least two directly successive time intervals, in this case in three directly successive time intervals 62a-c, reaches or exceeds a limit value. In principle, it is conceivable here to use different limiting values for each of the time intervals.

In all of the above methods, the limit value, at which the presence of particles 12 is inferred when the limit value is reached or exceeded, can be dependent on the desired background signal (noise).

Fig. 5 to 8 relate to an embodiment in which the detector 26 comprises, by way of example, a photodiode 26.1 which provides a continuous measurement signal 58. However, it is also possible (fig. 9) for the detector 26 to comprise an MPPC which provides discrete measurement signals in the form of a plurality of single-photon pulses 58. In this case, a particle 12 is considered to be detected when the number of single photon pulses 58 counted in the time interval 62 reaches or exceeds a limit value. Here, too, the width of the time interval is adapted in dependence on the velocity of the fluid.

The above generally described method for detecting particles 12 will now be explained with reference again to fig. 10: after the start in block 68, laser light 10 from laser 18 is focused into spot 22 in block 70. In block 72, a fluid, i.e. the exhaust gas 48 containing the particles 12, is guided through the light point 22 by means of the flow 49. In block 74, the thermal radiation 14 emitted by the spot 22 is detected by means of the detector 26. In block 76, the duration 64 of the time intervals 62a-c is determined, and in particular, the duration 64 of the time intervals 62a-c is determined based on the velocity of the flow 49 of exhaust gas 48 provided in block 78.

As described above, the detector 26 supplies the measurement signal 58, which is evaluated in its entirety in an evaluation block 80, which is illustrated by a dashed line. In detail, in each time interval 62a-c, the number of single photon pulses 58 within each time interval 62 is integrated (in the case of a continuous measurement signal 58) below the measurement signal 58, or (in the case of a discontinuous measurement signal 58) in block 82. In block 84, the determined integral or the determined number is compared with a limit value. If the limit is reached or exceeded, the particle 12 is considered detected in block 86. If, on the other hand, the limit value is not reached, it is assumed in block 88 that no particles 12 have been detected. The method ends at block 90.

The exhaust gas 48 is only an example of a possible measurement gas. The measurement gas can also be other gases or gas mixtures. The method can also be used in other scenarios and application areas (e.g. portable radiation monitoring systems, indoor air quality measurements, radiation of burning equipment (private, industrial)).

In the particle sensor shown, the laser light and/or the thermal radiation can also be conducted entirely or partially by means of an optical waveguide.

It is additionally conceivable to use the method in any HV corona sensor (HV-Korona-sensor) which is intended to measure the particle/aerosol concentration in the gas.

Claims

1. A method for detecting particles (12) or aerosols in a flowing fluid (48, 49), the detection being performed by using the principle of laser-induced incandescent light, characterized in that the method comprises the steps of:

a. focusing laser light (10) emitted from a laser (18) into a spot (22),

b. directing a fluid (48, 49) containing the particles (12) or aerosol through the light spot (22),

c. detecting thermal radiation (14) emitted from the light spot (22) by means of a detector (26),

d. the parameter (58) which is provided by the detector (26) and which characterizes the detected thermal radiation (14) is evaluated in a time interval (62), wherein the duration (64) of the time interval (46) is dependent on the speed of the fluid (48, 49).

2. The method of claim 1, wherein at least some of the time intervals (62) overlap.

3. Method according to claim 2, wherein the duration (64) of the time interval (62) is greater than a desired FWHM (60) of the quantity (58) characterizing the thermal radiation, in particular about 1 to 2 times the desired FWHM, more preferably about 1.5 times the desired FWHM.

4. The method according to at least one of claims 2 or 3, characterized in that the overlapping time period (66) corresponds to at least half of the duration (64) of the time interval (62).

5. Method according to at least one of claims 2 to 4, characterized in that particles (12) are detected when the quantity characterizing the thermal radiation (14) or the quantity derived from the thermal radiation reaches at least one limit value or a plurality of different limit values within a time interval (62).

6. Method according to claim 1, characterized in that at least some time intervals (62) successive to each other do not overlap, preferably directly adjoin each other.

7. Method according to claim 6, characterized in that a particle (12) is detected when the quantity characterizing the thermal radiation (14) or the quantity derived from the thermal radiation reaches at least one limit value in at least two immediately successive time intervals (62).

8. The method according to any one of claims 5 or 7, wherein the limit value depends on a desired background signal.

9. Method according to at least one of the preceding claims, characterized in that the quantity characterizing the thermal radiation (14) is a continuous quantity (58), preferably integrated from the continuous quantity within the time interval (62).

10. Method according to at least one of the preceding claims, characterized in that the quantity characterizing the thermal radiation (14) is a discontinuity quantity (58), which is preferably formed by pulse-like signals, the pulse-like signals (58) preferably being summed over the time interval (62).

11. Method according to at least one of the preceding claims, characterized in that the velocity of the fluid is found by the FWHM of preferably large particles, and the found velocity is then used to determine the length of the time interval for the detection of small particles.

12. Computer program, characterized in that it is programmed for implementing a method according to at least one of the preceding claims.

13. An electrical storage medium for an evaluation device (56), in particular for use in an exhaust system of an internal combustion engine, characterized in that a computer program for carrying out the method according to at least one of claims 1 to 10 is stored on the electrical storage medium.

14. A state machine, in particular in the form of an ASIC, characterized in that it is programmed for implementing a method according to at least one of claims 1 to 10.